After the introduction of immunisation against measles, mumps, and rubella, numerous outbreaks of mumps were reported in the 1980s and ‘90s in Switzerland and southern Europe.12 The Rubini strain is still widely used in Europe,3 and we report here a large outbreak of mumps in a population with a high vaccination rate and examine the differential efficacy of the three vaccine strains.

Participants, methods, and results

An outbreak was investigated in a small village in Switzerland. All children (ages 5-13) were included in the cohort. Information on immunisation status was obtained from vaccine certificates. The person who investigated the cases of mumps was blinded with regard to the vaccination status. A case was defined if mumps virus was isolated on culture, if a doctor confirmed the diagnosis, or if the typical clinical picture was described in a sibling of a patient with confirmed disease. The absence of IgG antibodies to mumps virus served as confirmation of full susceptibility to mumps in non-vaccinated children without clinical signs of the disease.

The cohort comprised 165 children. All questionnaires sent to their parents were returned and evaluated (response rate 100%). All immunised children had received their immunisation before the age of 2 years, almost half with the Rubini strain (table). Sixty six cases of epidemic parotitis occurred, resulting in an overall attack rate of 40%. Altogether 11(16%) children had parotid enlargement without fever; only one case (vaccinated with the Rubini strain) had a complicated course that required hospital admission. The attack rate was similar in unvaccinated children (63%) and children vaccinated with the Rubini strain (67%) but significantly lower in those vaccinated with the Jeryl-Lynn (14%) or the Urabe strain (8%)(table). When the attack rate for the two currently available vaccine strains was compared the relative risk of developing mumps was 4.8-fold greater (95% confidence interval 2.1 to 11.1) in children vaccinated with the Rubini compared with the Jeryl-Lynn strain. The low vaccine efficacy of the Rubini strain was observed throughout all age groups. In contrast, cases of mumps in children vaccinated with the Jeryl-Lynn or Urabe strains occurred only at the age of 8 or older. In the three vaccine categories no difference in the severity of mumps was observed.

Comment

More than a decade after systematic vaccination was introduced, the incidence of mumps is still high in Switzerland, Spain, and Italy. Several explanations for this are under discussion: inadequate vaccination rates, natural periodicity, and other factors such as differences in viral strains and loss of mucosal immunity. This study is notable because it describes an outbreak in a rural population with a high vaccination rate (95%). The attack rate of 63% in the unvaccinated group is consistent with other published reports. When compared with no vaccination, immunisation with the Rubini strain resulted in no detectable benefit.

Several serological surveys show comparable seroconversion rates for the Rubini, Jeryl-Lynn, and Urabe strains, but under field conditions other variables might be more relevant. This study supports the general importance of postmarketing surveillance.

To eliminate a disease a vaccination programme must achieve a high coverage with a vaccine that results in a substantial (>85%) vaccine efficacy.4 The Rubini strain investigated in this study clearly did not fulfil the second requirement. From a public health perspective, immunisation against mumps with the Rubini strain should be strongly discouraged unless the field efficacy of this vaccine is convincingly shown.

A second vaccination dose is generally recommended between the ages of 7 and 10 years. Other countries, such as Finland, eliminated indigenous mumps by instituting a two dose regimen with the Jeryl-Lynn strain.5 In agreement with this recommendation, we found no cases of mumps in children under 7 vaccinated with the Jeryl-Lynn or Urabe strain.

Acknowledgments

We thank the parents and children for providing the information for this study. We especially thank Diane Feldman for editing the manuscript, Dr Thomas Ammann (general practitioner) for the clinical data collection, Dr Hanspeter Zimmermann for general advice, Dr Christoph Minder for statistical advice, Dr Detlev Schultze and Edith Zwicky for laboratory support, and Isabella Brenner for logistical support PLV is supported by a grant from the Swiss National Foundation (3233-48902.96).

Contributors: MS was the primary investigator involved in the discussion of the core ideas, study design, development of the questionnaire, data collection, and analysis and wrote the manuscript. PLV initiated the study; participated in the study design, data documentation, and analysis; and contributed to the writing of the paper. JJO discussed core ideas, participated in the study design, performed the statistical analysis, and edited the paper. RLG discussed core ideas, participated in the protocol design and interpretation of the data, and edited the paper. PLV is guarantor for the study.